Estimating mixed layer depth and heat content at the end of sea ice melt season in the western Arctic Ocean

The summer surface mixed layer (ML) in the seasonal ice zone of the Arctic Ocean is reaching unprecedentedly high temperatures. This heat has to be released from the upper ocean over the course of the fall season before ice can grow on the surface. Estimates of upper ocean heat content are the first step in forecasting freeze-up, which is important for operational uses of the ice-free Arctic Ocean.

Two factors determine the heat content for the summer mixed layer of a water column: the depth of the surface mixed layer and the temperature of the seawater. Local mixed layer depth is estimated using an empirical relationship between ML depth found using CTD profiles and buoy measurements in the Chukchi and western Beaufort regions and local meteorological conditions from reanalysis products. Observations of ML depth (defined as the depth of the thermocline nearest the surface) show significant seasonal and inter-annual variability. Accounting for wind-driven and convective mixing through the empirical relationship improves estimates of mixed layer depth significantly over the standard 20-meter assumption. These depth estimates, combined with sea surface temperature products, enable estimates of surface mixed layer heat content in the region in late summer over the last several years (2010 – 2014).